The United Nations has called for an independent investigation into the Saudi-led coalition airstrike on Yemen's Houthi rebels, who say it killed at least 50 people and wounded 77, many of them children. The rebels say one of the missiles struck a bus taking children back from a summer school picnic.

U.N. Secretary-General Antonio Guterres urged an 'independent and prompt investigation' of the airstrike.

Mohammed Ali, a senior Houthi leader, said on Twitter that the Houthis welcomed Guterres call for the probe and are 'ready to cooperate.'

This image made from video taken on Thursday, Aug. 9, 2018, shows a child injured in an airstrike resting at a hospital in Saada, Yemen.

​Turning point

UNICEF chief Henrietta Fore said in a statement Friday that the 'horrific'attack 'marks a low point' in Yemen's brutal war. 'The question now is whether it will be a turning point -- the moment that must finally push the warring parties, U.N. Security Council and international community to do what is right for children and bring an end to this conflict.'

'I so hope that all parties will engage constructively in the political process,' said the U.N. special envoy to Yemen, adding 'including consultations scheduled in Geneva in September.'

Television pictures from a hospital showed blood-covered youngsters who seemed too stunned to even cry. Others writhed on the floor waiting for help.

Condemnation

'Grotesque, shameful, indignant,' the head of the Norwegian Refugee Council Jan Egeland tweeted. 'Blatant disregard for rules of war when a bus carrying innocent schoolchildren is fair game for attack.'

Save the Children said it 'condemns this horrific attack and is calling for a full, immediate, and independent investigation into this and other recent attacks on civilians and civilian infrastructure.'

Doctors Without Borders decries that 'civilians continue to pay the highest price' in Yemen while the regional director of the U.N. Children's Fund asked 'does the world really need more innocent children's lives to stop the cruel war on children in Yemen?'

The United States backs the Saudi-led coalition targeting Iranian-supported Houthi rebels in Yemen.

'We trust when they say that they will investigate. We closely coordinate with them,' State Department spokeswoman Heather Nauert said.

An injured child rests in a hospital a day after an airstrike in Saada, Yemen on Aug. 10, 2018.

​Tit for tat strike

Thursday's airstrike was carried out against a rebel-held area in Saada province, near the Saudi border. Missiles struck a market.

It was in response to a Houthi missile strike on Saudi territory Wednesday. Saudi defenses intercepted the missile, but fragments fell to the ground, killing one and wounding 11.

The coalition called Thursday's airstrike against the Houthis 'a legitimate military operation...carried out in accordance with international humanitarian law.'

The Saudis have accused the Houthis of using children as human shields, a charge the rebels deny.

Iranian-backed Houthi rebels seized the Yemeni capital of Sana'a in 2014, sending the Western-supported government into exile in Saudi Arabia.

The Saudi-led coalition began its air and ground campaign to drive out the Houthis more than three years ago. Its airstrikes have obliterated entire civilian neighborhood, including schools and hospitals, and compounded the misery of what is one of the world's worst humanitarian disasters.

Many Yemenis are in the brink of starvation. There are also severe shortages of fresh water and medicine. A cholera epidemic has killed thousands over the last two years.